1. Technical Field of the Invention
This invention relates to the manufacture of wooden roof trusses, of the kind including a bottom chord and at least one upper chord obliquely disposed in relation to the bottom chord, webs being connected between these chords.
2. Description of the Prior Art
In the manufacture of a set of such roof trusses it is customary to calculate, with the aid of a computer program, the composition of each truss according to span and loading, in the course of which the length and end cut geometry of each web, and its position in a truss, are determined. In a given set of trusses for a roofing job, there will be a great number of different webs which must be cut and correctly located during assembly.
We have found that it is possible to produce structurally satisfactory trusses in the varying sizes and types required in roofing construction, in which the webs are selected from a set of standard stock web lengths, with the panel points being determined by the successive selection of web lengths from stock lengths, and in which the ends of the webs are provided with a standard shape, set without regard to the geometry of the particular joints at which the ends are actually to be used.
In this way webs may be manufactured in advance of use and drawn from stock according to the specifications of a given job, with resultant cost savings.
In the preferred form of such webs, the ends are formed as semi-circles with a diameter substantially corresponding to the width of the web and their centres on the axis of the web.
U.S. Pat. No. 3,867,803 discloses a parallel chord joist truss in which the webs have such standardised end shapes. Unlike parallel chord trusses, however, the production of gable trusses and other roof trusses having oblique chords by the use of known procedures requires the production of webs of many specific lengths for a given job.
In its second aspect, the present invention provides methods of choosing a satisfactory web combination using the stock web lengths available, to achieve the desired structural performance. Whereas in the conventional approach, the panel points of the truss have been determined by the geometry of the chords (for example, where, in a so-called Fink truss, the panel lengths of the top chords are equalised, as are the panel lengths of the bottom chord, thus specifying the position of the panel points), in accordance with the present invention the actual panel points are determined by choosing webs from the stock lengths according to a predetermined scheme. The preferred schemes described herein relate the actual panel points defined by web selection to the panel points defined by panel length equalisation, for example by choosing a web length which will locate the actual panel point most closely on a predetermined side of the relevant notional panel point or by choosing webs which will contact the chords within provisionally determined panel point zones on the chords of the truss, the panel point zones being chosen on the basis of structural considerations.
In one such approach the invention provides a method of manufacturing wooden roof trusses of the kind having a bottom chord and at least one upper chord obliquely disposed relative to the bottom chord, the upper and bottom chords being connected by webs by means of nail-plated joints, characterised in that at least some of the webs are selected from a set of standard stock web lengths, and in that the ends of those webs are provided with a standard shape, set without regard to the geometry of the joints at which the ends are to be used.
Preferably the stock of webs comprises a set of web lengths which increase by equal increments between minimum and maximum lengths.
Preferably also the panel points of the truss are determined by the successive selection of web lengths from said stock.
In a particularly preferred form of the methods of the invention, the method includes the steps of:
a) determining notional panel points on the chords said panel points being joined by notional web lines;
b) choosing a starting point on a chord, said chord being a starting chord; and,
c) using successive webs of lengths chosen from said set of web lengths to form two alternating sets of webs, such that:
(i) in the case of one set of alternate webs, each web has its length chosen from said set of web lengths as the length the longest not greater than the length of a notional web having said predetermined end shape and fitting the distance from the joint of the web with the starting chord to the notional panel point for that web on the chord opposite the starting chord; and,
(ii) in the case of the other set of alternate webs, each web has its length chosen from said set of web lengths as the length the shortest which is at least the length of a notional web having said predetermined end shape and fitting the distance from the connection of said web with the said opposite chord to the inter-section with the starting chord of a line passing through said connection and parallel to the notional line of the web.
Preferably in such a method where the length thus determined of a web of said other set of alternate webs is greater than the said distance from the connection of said web with said opposite chord to the intersection with the starting chord of said line, the joint of the second web with the starting chord is located on the side of said intersection remote from the starting point.
Preferably also the starting point is the apex of the truss, and the notional panel points of an upper chord are determined by dividing the chord into panels, preferably equal panels, and the notional panel points of the bottom chord are at the intersection with the bottom chord of lines, preferably normal to the upper chord, intersecting the upper chord at the notional panel points thereof.
In another approach to the criteria for web selection, target panel point zones are established on structural principles, and alternative methods used to determine an efficient web layout within the constraints of such zones. Such a method may include the steps of
a) determining the maximum allowable panel lengths for each chord of the truss
b) determining the minimum number of overlapping maximum panel lengths in each chord, the regions of overlap thereof being referred to herein as panel point zones
c) selecting webs from the stock range on the basis of chosen criteria including the requirement that the chosen web will connect with the chord within a target panel point zone previously determined for the next web-to-chord joint.
Each of these alternative methods is characterised by a stepwise procedure reiterated where necessary, in which, starting at a chosen point on the truss, preferably either
(a) starting at the apex (or other point where the angle of the chord changes) and working toward the heels of the truss, or
(b) starting at the heels and working toward the apex, or
(c) starting with the provision of a king post and starting other web selection from the foot of the king post and working toward the heels
webs are selected from the stock range on the basis of chosen criteria including the requirement that the chosen web will connect with the chord within a target panel point zone previously determined for the next web-to-chord joint.
One of these alternative methods begins by establishing all possible combinations of webs satisfying the requirement that they begin and end in a panel point zone. A choice is then made between these combinations on the basis of a pre-determined criterion or set of criteria. Suitable criteria for this purpose include
(a) the sum of the individual departures of the panel points from the location of corresponding panel points in a standard solution, and
(b) timber usage.
Other criteria may be adopted.
In the second of the alternative methods, webs are chosen on the basis of a parameter extreme, for example,
(a) the web which hits the target panel point zone at a position within that zone closest to the starting point (eg. the apex or centre of the truss),
(b) the shortest stock length which will reach the target panel point zone,
c) the longest stock length which will reach the target panel point zone.
Other possible bases for web choice include choosing the web for which the included angle between the web and the chord is closest to a preferred angle for the panel point in question.
As a matter of convenience, the invention will be described primarily in its application to symmetrical trusses having a pair of upper chords meeting at a centrally located apex, but it is to be borne in mind that the invention may be applied, with appropriate modifications, to other types of trusses. Also as a matter of convenience, the trusses will be dealt with in the description of assembly methods in terms of single dimension line drawings, thereby avoiding the need to be concerned with web width and end shape, and with the choice of measurement conventions, for example the choice between the use of internal or external triangles. As is well known, various conventions are used in defining truss dimensions, particularly chord length. Providing the structural implications of the convention employed are taken into account, however, the choice of convention is of no relevance to the present invention.